1. Field of the Invention
The present invention relates generally to hermetic feedthrough assemblies, preferably of the type incorporating a filter capacitor. More specifically, this invention relates to a method for producing lower cost lead wires for use in hermetic feedthrough filter capacitor assemblies. Preferably, the feedthrough filter capacitor assemblies are of the type used in implantable medical devices such as cardiac pacemakers, cardioverter defibrillators, and the like, to decouple and shield internal electronic components of the medical device from undesirable electromagnetic interference (EMI) signals. The feedthrough assemblies provide a hermetic seal that prevents passage or leakage of fluids into the medical device. The lead wires and, consequently the hermetic feedthrough assemblies, are considerably less expensive than those made by the prior art using platinum, platinum/iridium lead wires while still achieving the same benefits of biocompatibility, providing good mechanical strength and achieving a hermetic feedthrough seal.
2. Prior Art
Implantable hermetic feedthrough assemblies are typically manufactured with lead wires composed of platinum or a combination of platinum and iridium. The platinum or platinum/iridium material is biocompatible and creates a hermetic seal through a gold brazing process that seals any gap between the lead wires and the ceramic substrate. The use of platinum and platinum/iridium lead wires also provides for good mechanical strength, which adds to the durability of the feedthrough. However, platinum is a precious metal that creates a manufacturing cost barrier.
Materials other than platinum and platinum/iridium with suitable mechanical properties and biostability can easily be found, but suffer from poor oxidation resistance, which can result in poor electrical conduction, poor weldability and poor solderability. In order to overcome these shortcomings, the prior art has developed several techniques for applying a layer of noble metal to a non-noble metal wire core. In the case of a mechanically clad coating having a typical thickness of from 5,000 nm to 20,000 nm, the coating material is not well adhered to the wire core, which causes problems with brazing and welding.
Relatively thick coating layers can also be produced by vacuum deposition techniques. However, they produce coatings that are subjected to relatively high stresses. Therefore, the practical limitation of a vacuum deposition coating is about 2,000 nm to 5,000 nm thick.
Another prior art process involves coating a relatively thin layer about 100 nm to 1,000 nm thick on the surface of a non-noble metal wire core. While such relatively thin coatings do not suffer from the stress forces inherent in thicker coatings, regardless how they are adhered to the wire core, these relatively thin coating thicknesses are insufficient to act as a barrier to migration of the non-noble metal to the surface of the wire. Subsequent operations such as brazing, welding and soldering can expose the non-noble material to oxidation, thereby causing a non-wettable surface that cannot be soldered.
Accordingly, there is a need for a relatively low cost lead wire for incorporation into hermetic feedthrough assemblies. The lead wire must be significantly lower in cost than those made from platinum and platinum/iridium without sacrificing biocompatibility, mechanical strength and ultimately the hermeticity of the feedthrough assembly into which it is built.